Inheritance And Selection

Question 1

🔴️Stabilising selection

Answer 1

1. occurs when the environment favours those with the most common (mean) characteristic
2. those on the extreme dies out
3. the frequency of the common characteristic increases
4. the range of characteristic (standard deviation) will reduce

Question 2

🔴️Despite being 25% likely to inherit a ff allele, why doesn’t this happen?

Answer 2

1 Random fusion of gametes

2 Small sample

3 Gametes not produced in equal numbers

4 Offspring ratios are probabilities not fixed

Question 3

Recessive allele

Answer 3

Only expressed in phenotype if homozygous;

Question 4

Co-dominant allele

Answer 4

If both present, then both are expressed in phenotype

Question 5

Why are genetic disease more prevalent in small, isolated populations?

Answer 5

1 Reproductively isolated so there is a small gene pool.

2 Inbreeding may occur so there is a high probability of mating with person having recessive allele

Question 6

Why is the actual popluation without the disease higher than predicted by the H-W calculation

Answer 6

1 People with the disease may die young so they may not reach reproductive age (adult)

2 There is a selective disadvantage

3 Genetic screening helps prevent genetic diesease being perpetuated.

Question 7

Describe and explain speciation

Answer 7

1 geographical isolation of organism;

2 Separate gene pool as no interbreeding between organisms

3 Different environmental conditions in each place

4 This means there are different selection pressures in separate populations;

4 Variation in population due to chance mutation occurs, causing organisms to acquire advanatageous allele

5 This advantageous phenotype makes indivudials more suited to their environment, and so more likely to survive and repoduce

6 The beneficial allele more likely to be passed on, increasing the frequency of the allele;

7 Over a long time, the populations have changed so much that they’re unable to produce fertile offspring

Question 8

Explain why Males are more likely to show a phenotype produced by a recessive allele carried on the X chromosome.

Answer 8

1 Males have one allele** for each gene so recessive alleles **always expressed in Male;

2 Females need two recessive allele (homozygous recessive)

3 females recessive alleles can be masked by dominant allele

Question 9

Explain Phenotype

Answer 9

• Expression of a characteristic
• due to genotype and environment;

Question 10

Dominant allele

Answer 10

characteristic Is always expressed in the phenotype when present

Question 11

What is a sex-linked gene?

Answer 11

a gene carried on one of the sex chromosomes, normally the X chromosome

Question 12

Directional selection

Answer 12

• When the environment favours those with a feature/allele as it confers advantage
• so more likely to survive and reproduce;
• The allelic frequencies increases so more individuals with feature

Question 13

Species

Answer 13

• group of organisms with similar physiological features;
• which can interbreed to produce fertile offspring;

Question 14

An allele may be present in the genotype but its effects are not seen in the phenotype. In terms of protein production, explain why

Answer 14

• Effect of recessive allele not seen when dominant allele present

This is true if the:

• Recessive allele codes for non-functional protein;
• Dominant allele codes for functional protein;
• Therefore the effect nullifies the recessive

Question 15

Homologous chromosomes

Answer 15

• Capable of pairing during meiosis
• Same gene loci;

Question 16

What’s the H-W principle

Answer 16

• The proportion of alleles** of a particular **gene
• Will stay constant from one generation to the next
• Providing no mutation, mating at random and no migration;

Question 17

How do you know a disease is caused by a recessive allele?

Answer 17

• 2 unaffected parents (X,Y) give birth to (Z) affected
• Parents must both be heterozygous

Question 18

How do you know if a disease is caused by a gene on the X chromosome

Answer 18

• Uaffected fathers (XY) give birth to affected sons
• Mother gives the X chromosome to son and father gives the Y

Question 19

Explain how mutation may produce multiple alleles of a gene.

Answer 19

• Changes base sequence
• In different places within gene;